1. Field of the Invention
The invention relates to trailers, and more particularly, to automatic steering mechanisms for trailers.
2. Description of the Prior Art
A wide variety of trailers are commonly used in the trailer rental industry. In the past, many automobile owners have had relatively large, powerful automobiles which they occasionally used for towing rental trailers. Since the weight of such rental trailers, when loaded, has often been substantially less than the weight of the tow vehicle, the brakes of the tow vehicle have been adequate to stop both the tow vehicle and the loaded trailer, thereby obviating the need for providing trailer brakes. When it has been necessary to provide brakes on a trailer, a type of brake device known as a mechanical surge brake device has been used. Mechanical surge brake devices are connected in series relationship with the tongue of the trailer and the hitch ball of the towing vehicle. When the brakes of the towing vehicle are applied, inertia of the loaded trailer in a forward direction causes compression of the mechanical surge brake device, shortening the effective length of the tongue by approximately one to two inches, depending on the state of adjustment of the trailer brakes. The compression of the mechanical surge brake device actuates the hydraulic brake system of the trailer, applying the brake as long as the forward force continues to be applied by the inertia or weight of the trailer against the hitch ball of the towing vehicle. Various mechanical surge brakes devices often include brake drum mechanisms which deactuate the wheel brakes when the towing vehicle backs up and pushes the towed trailer in the reverse direction, thereby allowing the trailer to be "backed up" without causing its brakes to remain actuated due to the compressive force applied to the mechanical surge brake device by the trailer hitch.
It is well known that there is trailer-to-car oscillation that occurs when a car tows a trailer. This type of oscillation must be controlled and adequately "damped" in order to prevent dangerous and undesirable instability of the trailer and tow vehicle, especially at high speeds. In the past, a very important technique for reducing the amplitude of such oscillations simply has been to increase the "tongue weight", i.e., the downward force exerted by the tongue of the trailer on the tow vehicle. This approach has been relatively satisfactory in reducing the amplitude of trailer-to-car oscillations to non-dangerous levels only because the average automobile used for towing trailers has been quite heavy compared to the weight of the loaded trailer. However, in recent years there has been a trend to reducing the size and weight of most automobiles. Many compact or subcompact automobiles are small, relatively light, and have front wheel drive. These factors greatly reduce the amount of tongue weight that can be safely applied to trailer hitches installed on such cars. Thus, reducing of the above mentioned dangerous instability caused by car-to-trailer oscillations by merely increasing the tongue weight is no longer always satisfactory, because of the inability of today's smaller automobiles to support great tongue weights. It therefore has become necessary to find alternate ways of improving the stability of trailers having with relatively low tongue weights, especially for trailers designed for rental to consumers.
Accordingly, it is an object of the invention to provide a highly stable trailer which has a relatively low tongue weight on the towing vehicle.
In order to provide a suitable degree of stability and safety for a relatively large, heavy trailer, and in order to also accomplish the objective of low tongue weight, it has been found to be expedient to use so-called "spread axle" trailers, wherein separate axles are provided close to the front and rear portions, respectively, of the trailer. This leads to the requirement that the front wheels of the spread axle trailer be steerable as the trailer is towed or backed up around sharp curves by the tow vehicle. Numerous devices have been proposed for accomplishing steerable front wheels on four wheel trailers. Many such devices have had the serious shortcoming that the control mechanism for pivoting the front wheels of the trailer to accomplish steering accomplished such pivoting in response to lateral swiveling of the trailer tongue relative to the trailer frame. Such systems result in trailers that are very difficult for amateurs to control while backing up because "jack-knifing" usually occurs as a result of slight steering errors by the driver. It is essential that trailers rented to the general public be capable of being easily backed up without jack-knifing.
It is another object of the invention to provide a safe, highly stable spread axle trailer that is easily backed up by an average automobile driver and does not cause jack-knifing as a result of minor steering errors.
Due to the recent prevalence of small automobiles, it is becoming increasingly necessary to provide automatic braking systems on rental trailers. Electrical connectors which are frequently disconnected and re-connected, as is necessary in the trailer rental business, are inheritently unreliable and also are subject to frequently being damaged by the renters. This causes use of electrical surge braking systems to be unsatisfactory for use on rental trailers. Consequently, only mechanical surge braking devices are satisfactory for rental trailers. However, use of present mechanical surge brake systems results in a substantial shortening of the effective tongue length when the trailer brakes are actuated by a mechanical surge brake device as a result of applying the brakes of the towing vehicle when it is moving forward. It is necessary that an automatic steering system for a spread axle trailer be not undesirably influenced by the above mentioned shortening of the effective tongue length as a result of activation of a mechanical surge brake device.
Accordingly, another objective of the invention is to provide a highly stable spread axle trailer having a steering system which is not unduly sensitive to changes in the effective tongue length of the trailer occuring when the brakes of a tow vehicle are applied.
A number of trailer steering devices have been proposed which either avoid (or could be adapted to avoid) the above mentioned tendency to "jack-knife" as a result of minor steering errors made by the tow vehicle driver while "backing up". However, most of the known steering devices utilize rigid connections between the tow vehicle and the trailer. This results in potentially dangerous changes in the "self-steering" trailer when the brakes of the tow vehicle are applied, or else results in a great loss of accuracy in self-steering control when the towed vehicle is backed up if a mechanical surge brake mechanism is used.
For example, U.S. Pat. No. 1,422,040 discloses a trailer steering device in which the tongue pivots only in the vertical direction, and therefore, remains aligned with the trailer. However, an impractical scissor-like steering mechanism is used to accomplish pivoting of both the front and rear wheels to accomplish steering of the trailer as the two vehicle traverses a turn. An energy absorbing spring incorporated in the hitch mechanism allows the trailer to move forward slightly, relative to the tow vehicle, when the brakes of the tow vehicle are applied. The scissor-like mechanism then opens slightly, greatly reducing the sensitivity of the steering mechanism during backing up of the trailer. This causes undesirable mis-steering by the mechanism and undesirable sideways "scrubbing" of the trailer wheels and tow vehicle wheels on the road as sharp turns are negotiated. U.S. Pat. Nos. 4,191,394, 4,171,825, and 3,876,240 all disclose steering devices which rely on an auxiliary hitch point attached to the tow vehicle for detecting changes in direction of the tow vehicle, relative to the trailer, and causing corresponding pivotal self-steering of the front wheels of the trailer. All of the disclosed systems would result in dangerous and abrupt changes in the direction of self-steering and consequent lateral lurching and applying of lateral forces on the light rear end of the tow vehicle when the brakes of the tow vehicle are suddenly applied. This is especially true at high speeds, thereby creating potentially very dangerous situations. U.S. Pat. No. 2,498,779 discloses a self-guiding trailer dolly which, if modified to incorporate a mechanical surge brake system, would provide forward steering control that would be relatively insensitive to changes in the effective tongue length resulting when brakes of the tow vehicle are applied. However, that system is inheritently very unstable due to the fact that the entire front axle is pivotally connected at its center point to the frame of the trailer and pivots at the center point to accomplish steering; this structure prevents the front wheels and axle support system from effectively limiting sideways oscillation of the trailer. U.S. Pat. No. 3,753,580 discloses a steering system designed to cause a two wheel trailer to track properly. In one disclosed embodiment, a telescoping linkage from the tow vehicle to the steering mechanism is provided. However, the telescoping arrangement greatly decreases the degree of control over steering of the trailer that can be accomplished, because while the trailer is being towed, there will be no steering control for a substantial amount of "turning" (of the trailer and tow vehicle around a curve) in one direction, and while backing up, there will be no steering control of a substantial amount of "turning" in the other direction.
Accordingly, it is another object of the invention to provide a highly stable, steerable four wheel spread axle trailer which overcomes the shortcomings of the known prior art.